1. Field of the Invention
The present invention relates to a transmission apparatus and a peak reduction method. In particular, the present invention relates to a transmission apparatus for CDMA (Code Division Multiple Access) and a method for reducing the amplitude peak in its transmission signal.
2. Description of the Related Art
FIG. 8 is a block diagram showing an example of a configuration of a conventionally known transmission apparatus for CDMA terminal. In FIG. 8, data input to a transmission apparatus 100 are transmission data D1 to D6 and control data C0 to C2. The number of actually used transmission data and the number of control data might be decreased according to the subject apparatus. Spreaders 100a, 100b, 100c, 100d, 100e, 100f, 100g, 100h and 100i respectively spread the input transmission data D1, D3 and D5, control data C1, transmission data D2, D4 and D6, and control data C2 and C0 in accordance with a spreading factor specified by an upper layer which is not illustrated. The transmission data D1, D3 and d5 and the control data C1 are assigned to the Ich (In-phase channel), whereas the transmission data D2, D4 and D6 and the control data C0 and C2 are assigned to Qch (Quad-phase channel).
An adder 101a is an adder for conducting addition for Ich. The adder 101a adds up signals output from spreaders 100a, 100b, 100c and 100d. An adder 101b is an adder for conducting addition for Qch. The adder 101b adds up signals output from spreaders 100e, 100f, 100g, 100h and 100i. 
A scrambling circuit 103 generates a scrambling code for conducting spreading modulation. A spreading modulation circuit 102 is supplied with a scrambling code which is an output of the scrambling circuit 103, and the spreading modulation circuit 102 conducts spreading modulation on data respectively input from the adder 101a and the adder 101b. 
Band limiting filters 104a and 104b are filters for conducting band limiting on signals respectively of the Ich and Qch subjected to spreading modulation and output from the spreading modulation circuit 102. In order to raise the frequency utilization efficiency in an analog baseband signal when generating a transmission signal, band limiting filters are needed, therefore, the band limiting filters 104a and 104b include LPFs (Low Pass Filters) for conducting spectral shaping on the transmission signal. In terminal stations, for example, root raised cosine filters having a roll off factor of 0.22 are used as the LPFs.
DA converters 106a and 106b are digital-analog converters for the Ich and Qch, respectively. The DA converters 106a and 106b convert signals output from the band limiting filters 104a and 104b to analog signals, respectively. A modulation circuit 107 is a circuit for conducting analog modulation (QPSK: Quadrature Phase Shift Keying) on the Ich signal and Qch signal converted to analog signals by the DA converters 106a and 106b. An RF circuit 108 has a function of converting the analog signal modulated by the modulation circuit 107 to a carrier frequency and a function of amplifying a transmission signal. An antenna 109 has a function of radiating the transmission signal output from the RF circuit 108 as an electromagnetic wave.
As for amplifiers in the modulation circuit 107 and the RF circuit 108 used in the transmission apparatus 100 having such a configuration, it is necessary to use A-class amplifiers having fine linearity and wide dynamic characteristics in order to amplify an input waveform without distortion. If multiplexing (multi-coding) of signals advances, a large peak occurs in amplitude and the PAR (Peak-to-Average power ratio) representing the ratio of the peak to the average power becomes large. Therefore, it is conceivable to conduct clipping on the amplitude in order to improve the efficiency of power conversion. By doing so, however, distortion occurs in the spectrum, resulting in influence upon adjacent channels.
By the way, in the field of the mobile communication system represented by cellular phone, drawing up and proposal of standards are being performed by 3GPP (3rd Generation Partnership Project). Among them, specifications of a UE (User Equipment) which is a cellular phone or a terminal station, such as provisions concerning leak power of adjacent channels, are defined. When forming a transmitter in a terminal station on the basis of specifications of 3GPP, the design should satisfy the provisions.
In the standards of 3GPP, multiplexing of transmission codes at a terminal station is allowed, however, if the multiplex number of codes increases, a large peak occurs in the signal amplitude in some combination of the transmission data sequence. It is known that the transmission data sequence that generates the peak occurs with some probability and the peak pattern have a relation to the transmission data sequence.
On the other hand, when generating a transmission signal in wireless communication, a filter is used to conduct band limiting on an analog baseband signal. As the band limiting filter, an LPF (low pass filter) for conducting spectrum shaping on the transmission signal is used.
Typically, the band limiting filter of this kind is implemented as a digital circuit using an FIR (Finite Impulse Response) filter in order to obtain linear phase characteristics. The FIR filter outputs a waveform of an impulse response as shown in, for example, FIG. 9. Each of output waveforms of the DA converters 106a and 106b is a waveform formed by impulse response waveforms of individual transmission data overlapping along the time axis. An example thereof is shown in FIG. 10. In order to facilitate illustration, FIG. 10 shows not a waveform at the time of a multi-code, but a waveform at the time of a single code. If impulse response waveforms each having a waveform as shown in FIG. 9 overlap at a peak such as a point P, a point Q and a point R, a large peak which exceeds range of planned target R, such as a peak at a point S or a point T shown in FIG. 10, occurs sometimes. As already described, such a peak occurs with some probability.
When designing the transmission circuit of a terminal station in the mobile communication system, therefore, the number of bits in the DA converter 106a and 106b are determined with due regard to the characteristics of the filter so as to be capable of representing the entire waveform even if a peak exceeding the range of planned target occurs as represented by the point S and T in FIG. 10. For conducting the design with such a number of bits, however, a wide range is demanded for the frequency handled by the DA converters 106a and 106b. Furthermore, it also becomes important to take the CCDF (Complementary Cumulative Distribution Function) and the PAR into consideration in representing the amplitude of the transmission signal.
In other words, if the bit width of the DA converters 106a and 106b is determined so as to represent all amplitudes, the ratio of the peak to the average power becomes large and consequently it becomes necessary to take the influence of the quantization noise as well into consideration. In addition, there is also a problem that the circuit scale becomes large.
If there is a large peak in the transmission signal input to an amplifier in the succeeding modulation circuit 107 or RF circuit 108, a nonlinear region is used and consequently there is a fear that leak power to adjacent channels will be aggravated. For preventing this, it is demanded to use an A-class amplifier having a fine linearity and a wide range as described above. However, it leads to an increased cost of the apparatus.
In the circuit configuration of the transmitter in the conventional terminal station, the code multiplex number is comparatively small and consequently the circuit is designed so as to be able to represent all amplitudes without being much conscious of peaks. Recently, however, an increase of quantity of data transmitted to the base station on an uplink is expected. Furthermore, if multi-coding is adopted in the terminal station as well, it is considered that the amplitude peak in the transmission signal as described above will become larger. Therefore, means for suppressing the peaks are demanded.
Some conventional apparatuses suppressing the amplitude peaks in the transmission signal are known. For example, in a peak reduction apparatus described in JP-A-10-271072 (FIG. 1), a code sequence having a predetermined length and having a pattern which increases in amplitude when a transmission code sequence is limited in band by a transmission filter is set in a register. The peak reduction apparatus includes a comparison unit for comparing the code sequence with a transmission code sequence shifted into a shift register, and a code amplitude reduction unit supplied with a comparison coincidence signal from the comparison unit to reduce amplitude of at least one code disposed in the center of a predetermined length of the transmission code sequence. According to the peak reduction apparatus described in JP-A-10-271072, spread of the envelope of the signal limited in band by the transmission filter is reduced. Therefore, spread of the band caused by nonlinear distortion in a transmission amplifier unit can be suppressed.
In a peak factor reduction apparatus described in JP-A-2003-124824 (FIGS. 1 and 3), a correction signal which concentrates energy only immediately in the vicinity of a peak pulse is generated. Elimination of the peak pulse is conducted on the basis of the correction signal. As a result, the influence on the degradation of the signal quality can be held down to a slight value. This apparatus inputs an input signal to a reference filter and predicts what kind of a peak occurs when band limiting is conducted. Only a part for which an output of the reference filter has exceeded a preset value is extracted by an amplitude control unit to form a peak pulse. Subsequently, at a point in time in which the peak pulse becomes the maximum, an impulse signal having amplitude proportionate to the peak pulse is generated. The input signal is delayed by a delay unit and aligned in timing with the impulse signal. The impulse signal is subtracted in signal from an output of the delay unit by an adder, and a resultant signal is output. The output signal is finally limited in band by a band limiting filter. On the basis of the superposition theorem in the linear circuit, the peak amplitude generated by the input signal and the impulse response amplitude generated by the impulse signal coincide with each other in position and amplitude, and they are opposite in phase. Therefore, an amplitude component that has exceeded the peak is suppressed, and the peak factor can be held down to a preset value.
In JP-A-2002-164799 (FIG. 2), a transmission power control method for communication apparatus capable of suppressing the occurrence of a peak factor at the time of user multiplexing is disclosed. In this method, transmission data before being input to a band limiting filter is branched, then one of the branched transmission data is passed through a peak detection filter having the same configuration as a band limiting filter to obtain a correction value for suppressing a power peak of the transmission data. The other of the branched transmission data is delayed by a time for obtaining the correction value, then corrected by the correction value, and then input to the band limiting filter. A maximum value of a power peak of transmission data passed through the peak detection filter is obtained every sampling time. The maximum value is compared with a peak suppression threshold to obtain a correction value.
By the way, it is desirable that an increase of signal delay caused by a circuit added to hold down the amplitude peak of the transmission signal is as small as possible. In the conventionally known techniques, however, a considerably long delay is caused by the added circuit.
In other words, in the peak reduction apparatus described in JP-A-10-271072, a delay time for shifting the transmission code sequence in the shift register is added before holding down the amplitude peak of the transmission signal in the code amplitude reduction unit.
In the peak factor reduction apparatus described in JP-A-2003-124824, the input signal is delayed by the delay unit to align the input signal in timing with the impulse signal, resulting in an increased delay time.
In the transmission power control method described in JP-A-2002-164799, the delay is increased by the time for obtaining the correction value.